Udo Rumpf wrote:

Hi,

Thank you all, I have learned a lot. I have also come up
with two more things to consider, both rather minor I suspect:

- The control linkages are propably more complicated
in a T-Tail (con).
- With a T-Tail you can build the elevator in one piece so you
can rig and derig more easily (pro).

Now why are we not seeing more V-Tails? The main pro for T-Tails
seem to be:

- Good ground clearance
- Less drag
- Operates in clean undisturbed air

How does a V-Tail stand up against that?

Ground clearance is not an issue (I speak from experience)
It is questionable as to it having less drag. The theory says yes.
In practise it is not as easy to design a V tail that can match the T tail.
The lower part of the vertical stab on a T tail is in disturb air as well.
In case of the V tail you would have two surfaces in the disturb air.
The mixer is a simple and light weight mechanical device. If built and
installed right cross interference is minimal.

Udo

The V-tail is inherently less efficient than tails with separate horizontal
and vertical surfaces (conventional and T tails) as a pitch stabilizer.
For any speed except maybe just one, the horizontal stabiliser has to provide
some vertical force in order that the glider remain trimmed. But in order to
obtain the same vertical force from a V-tail, the normal forces on both surfaces
need to be higher than the half of the total vertical force, because only their
vertical component is useful, there are also horizontal components which cancel
each other. But this increased normal force is lift and so produces an increased
induced drag.

Not very important if the V is very flat, but then the efficiency in yaw control,
i.e. as a rudder and fin, is poor and a similar argument may be developed: now we
are interested in the horizontal component and the vertical (higher) components are a
nuisance increasing induced drag. However no such component exists in steady straight
flight, so the inconvenience is less important.

It may happen that in a very well suited situation of steady turn the above argument
may be reversed in favor of V-tails: other tails need down elevator forces and outside
turn rudder force, the resulting force being closer to the horizontal direction than both
the preceding one could be provided with less induced drag by just one of the ruddervators
if properly oriented. But I think that in performance oriented designs the priority is
to minimize the drag in straight flight, and anyway this would be in favor of V-tail just
for (some range around) some very well suited bank angle and speed.
,